Volume 95, Issue 15, 12 October 2009
- lasers, optics, and optoelectronics
- plasmas and electrical discharges
- structural, mechanical, thermodynamic, and optical properties of condensed matter
- electronic transport and semiconductors
- magnetism and superconductivity
- dielectrics and ferroelectricity
- nanoscale science and design
- organic electronics and photonics
- device physics
- biophysics and bio-inspired systems
- interdisciplinary and general physics
Index of content:
The effects of surface energy on the wetting transition for impinging water droplets were investigated on the chemically modified nanowiresurfaces. We could modify the surface energy of the nanowires through chemisorption of alkyltrichlorosilanes with various carbon chain lengths and also by the ultraviolet-enhanced decomposition of self assembled monolayer molecules. Three surfacewetting states could be identified through the balance between antiwetting and wetting pressures. This approach establishes a simple strategy for design of the water-repellent surface to impinging droplets.
- LASERS, OPTICS, AND OPTOELECTRONICS
95(2009); http://dx.doi.org/10.1063/1.3245309View Description Hide Description
Air band modes of three-dimensional photonic crystals (3DPCs) have a higher photonic density of states, potentially enabling greater emission enhancement. However, it is challenging to introduce emitters into the “air” region without significantly disturbing the photonic band structure of the PC. Here, we overcome this difficulty by introducing a low refractive index aerogel matrix containing CdSequantum dots (625 nm peak emission) into a titanium dioxide logpile PC. We observe that the aerogel infiltration indeed preserves the bandstructure. We measure an emission suppression of times inside and an enhancement of approximately three times outside the bandgap with only one vertical unit cell.
95(2009); http://dx.doi.org/10.1063/1.3237179View Description Hide Description
We have shown that alloying a noble metal (gold) with another metal (cadmium), which can contribute two electrons per atom to a free electron gas, can significantly improve the metal’s optical properties in certain wavelength ranges and make them worse in the other parts of the spectrum. In particular, in the gold-cadmium alloy we have demonstrated a significant expansion of the spectral range of metallic reflectance to shorter wavelengths. The experimental results and the predictions of the first principles theory demonstrate an opportunity for the improvement and optimization of low-loss metals for nanoplasmonic and metamaterials applications.
95(2009); http://dx.doi.org/10.1063/1.3244203View Description Hide Description
Carrier transport in InGaNlight emitting diodes has been studied by comparing the electroluminescence(EL) from a set of triple quantum well structures with different indium content in each well, leading to multicolor emission. Both the sequence and width of the quantum wells have been varied. Comparison of the ELspectra reveals the current dependent carrier transport between the quantum wells, with a net carrier flow toward the deepest quantum well.
95(2009); http://dx.doi.org/10.1063/1.3246153View Description Hide Description
We show how one-dimensional structured media can be used to measure chirality, via the spectral shift of the photonic band gap edges. Analytically, we show that a chiral contrast can, in some cases, be mapped unto an index contrast, thereby greatly simplifying the analysis of such structures. Using this mapping, we derive a first-order shift of the band gap edges with chirality. Potentially, this effect could be used for measuring enantiomeric excess.
95(2009); http://dx.doi.org/10.1063/1.3246161View Description Hide Description
Photoluminescence from InPquantum dots within a GaP matrix is investigated as a function of magnetic field. By fitting the magnetoluminescence data to the energy states of the electrons and holes confined in quantum dots in a perpendicular magnetic field, the electron-hole reduced effective mass is determined to be . The diamagnetic shift in the luminescence peak gives an average exciton radius of about 5 nm, smaller than the dot radius. These results indicate that the recombining electrons are primarily composed of states originating from the InP valley although the conduction band discontinuity between the InP and the GaP barrier is expected to be very small or even negative.
95(2009); http://dx.doi.org/10.1063/1.3247888View Description Hide Description
ion-exchanged channel waveguideamplifiers have been fabricated in -doped acid-resistant aluminum germanate glasses. The optical and relative gains of a 3.15-cm-long waveguide channel were achieved to be 4.05 and 2.29 dB at wavelength under 110 mW 793 nm laser excitation, respectively. After compensating the propagation loss, an internal gain of 1.50 dB and a remarkable gain coefficient of 0.48 dB/cm were obtained, which reveals a definite -band signal amplification in the low phonon energy glass waveguide. As an expectation, UV-radiation-sensitive glass waveguide should promote the developments of gain-flatten -band waveguideamplifiers, infrared UV-writing grating waveguide lasers, and compact multifunctional integrated optical devices.
95(2009); http://dx.doi.org/10.1063/1.3242012View Description Hide Description
The electro-optic (EO) properties of monoclinic (BTM) single crystals are studied using the interference technique by applying the external electric field. The measured room temperature free EO coefficients at 632.8 nm are 10.2 and 9.0 pm/V for and , respectively. The EO effect of BTM is about three times as high as that of (KDP) crystal, which makes it a very promising candidate for the EO applications.
Effect of material properties on reverse flow in nematic liquid crystal devices with homeotropic alignment95(2009); http://dx.doi.org/10.1063/1.3242018View Description Hide Description
Reverse flow is undesirable in liquid crystal devices with vertical alignment. The influence of the material properties on the onset of backflow is investigated for commercially available negative dielectric liquid crystals. It is shown that the threshold voltage for the occurrence of backflow is an important materialcharacteristic. This threshold is relevant for applications and a large value is desired in devices to avoid backflow while keeping a wide applicable voltage range. Accurate finite element simulation of the liquid crystalhydrodynamics allows extraction of and the unknown Miesowicz coefficients . The resulting values are tabulated at .
Plasmon enhanced light emission from InGaN quantum wells via coupling to chemically synthesized silver nanoparticles95(2009); http://dx.doi.org/10.1063/1.3249579View Description Hide Description
Chemically synthesized single-crystal silvernanoparticles are used to demonstrate plasmon enhanced visible light emission from nitride semiconductor quantum wells. For ease of assembly and testing, the nanoparticles are embedded onto the surface of flexible resin films, which are then simply adhered on top of the light emitting samples. Large enhancements in photoluminescence efficiency are correspondingly measured at emission wavelengths near the nanoparticleplasmonic resonance. At the same time, when samples emitting at a sufficiently far detuned wavelength are used, the measured efficiency is not affected by the nanoparticles, which confirms the plasmonic origin of the observed enhancement.
Terahertz spectroscopy of shift currents resulting from asymmetric (110)-oriented GaAs/AlGaAs quantum wells95(2009); http://dx.doi.org/10.1063/1.3249611View Description Hide Description
We report the observation and the study of an additional shift current tensor element in (110)-oriented GaAs quantum wells, which arises from an out-of-plane asymmetry of the quantum well structure. The current resulting from this tensor element is optically induced with 150 fs laser pulses and detected by measuring the simultaneously emitted terahertz radiation. This terahertz spectroscopy of shift currents is a powerful technique for symmetry investigations, which shows, for example, that our nominally symmetric (110)-oriented GaAs/AlGaAs quantum wells grown by molecular beam epitaxy are in reality asymmetric structures with different right and left interfaces.
Förster resonance energy transfer enhanced color-conversion using colloidal semiconductor quantum dots for solid state lighting95(2009); http://dx.doi.org/10.1063/1.3222902View Description Hide Description
In this paper, we present Förster resonance energy transfer (FRET)-enhanced color-conversion using colloidalsemiconductor quantum dot nanocrystals (NCs) to make reddish-orange light-emitting diodes for use in ultraefficient solid state lighting. To achieve FRET enhancement at 614 nm, we use an energy gradient hybrid structure made of cyan- and orange-emitting CdSe/ZnS NCs ( and 588 nm in solution, respectively). This enables recycling of trapped excitons using FRET and achieves a relative quantum efficiency enhancement of 15.1% in reddish-orange full color-conversion for the integrated hybrid cyan-orange NC layer with respect to the case of full color-conversion using only orange NCs without FRET.
High power thermoelectrically cooled and uncooled quantum cascade lasers with optimized reflectivity facet coatings95(2009); http://dx.doi.org/10.1063/1.3246799View Description Hide Description
We present a method of preserving the device wall-plug efficiency by adjusting mirror losses with facet coatings for longer cavity quantum cascade lasers. An experimental study of output power and wall-plug efficiency as functions of mirror losses was performed by varying the front facet coatingreflectivity with a high-reflectivity-coated rear facet. The use of optimized reflectivitycoatings on -long chips resulted in continuous-wave output power of at for thermoelectrically cooled devices mounted on AlN submounts and average and continuous-wave output power in excess of for uncooled devices emitting at .
Enhancement of -type conductivity by modifying the internal electric field in Mg- and -codoped superlattices95(2009); http://dx.doi.org/10.1063/1.3248026View Description Hide Description
The internal electric field is modified by using Mg- and -codoped superlattices(SLs). The first-principles simulation results show that the internal electric field in SL has been significantly intensified due to the charge transferring from Si-doped interface to Mg-doped interface. Accordingly, the Mg- and -codoped -type SLs are grown by metalorganic vapor phase epitaxy and higher hole concentration as much as twice of that in modulation-doped SL has been achieved, as determined by Hall effect measurements. Furthermore, by applying Mg- and -codoped SLs with high Al content as the -type layers, we have fabricated deep ultraviolet light emitting diodes with superior current-voltage characteristics by lowering Mg-acceptor activation energy.
95(2009); http://dx.doi.org/10.1063/1.3251073View Description Hide Description
We report on the observation of the strong coupling regime occurring between a Tamm plasmon (TP) mode and an exciton from inorganic quantum wells(QWs). The sample is formed by a silverthin filmdeposited onto an AlAs/GaAlAs Bragg reflector containing InGaAs QWs located in the high refractive index layers. Angular resolved reflectometry experiments evidence a clear anticrossing in the dispersion relations, a signature of the strong coupling regime. The Rabi splitting energy is 11.5 meV. The experimental data are in very good agreement with simple transfer matrix calculations. The emission from low and high energy TP/exciton polaritons is also demonstrated.
Two-directional lasing from a dye-doped two-dimensional hexagonal photonic crystal made of holographic polymer-dispersed liquid crystals95(2009); http://dx.doi.org/10.1063/1.3251078View Description Hide Description
Lasing actions are demonstrated in two high-symmetry directions ( and ) of a two-dimensional hexagonal photonic crystal from dye-doped holographicpolymer-dispersed liquid crystals. The group velocity anomaly, which is peculiar to two-dimensional and three-dimensional photonic crystals, leads to substantial enhancement in local fields in the two-dimensional photonic crystal with a low-index contrast. The difference on the lasing thresholds between the two directions is interpreted as the result of different group velocities of the flat bands where laser oscillation occurred.
95(2009); http://dx.doi.org/10.1063/1.3243694View Description Hide Description
This paper reports on surface illuminated Gephotodetectors monolithically integrated on Si substrate operating in the and wavelength bands. The responsivity at a wavelength of ranges from 0.08 to 0.21 A/W without bias voltage for Ge mesa diameter ranging from 10 to , respectively. The measured −3 dB cut-off frequency is as high as 49 GHz under a reverse bias of 5 V at a wavelength of . An open eye diagram up to 40 Gbit/s is also demonstrated.
- PLASMAS AND ELECTRICAL DISCHARGES
CH spectroscopy for carbon chemical erosion analysis in high density low temperature hydrogen plasma95(2009); http://dx.doi.org/10.1063/1.3238295View Description Hide Description
The CH molecular band is measured upon seeding the hydrogen plasma in the linear plasma generator Pilot-PSI [electron temperature and electron density ] with methane. Calculated inverse photon efficiencies for these conditions range from 3 up to due to a steeply decreasing electron excitation cross section. The experiments contradict the calculations and show a constant effective inverse photon efficiency of for . The discrepancy is explained as the CH level is populated through dissociative recombination of the molecular ions formed by charge exchange. Collisional de-excitation is observed for and . These results form a framework for in situcarbonerosion measurements in future fusion reactors such as ITER.
95(2009); http://dx.doi.org/10.1063/1.3250157View Description Hide Description
Aluminum oxide layers can provide excellent passivation for lowly and highly doped-type siliconsurfaces. Fixed negative charges induce an accumulation layer at the -type silicon interface, resulting in very effective field-effect passivation. This paper presents highly negatively charged aluminum oxide layers produced using an inline plasma-enhanced chemical vapor deposition system, leading to very low effective recombination velocities on low-resistivity -type substrates. A minimum static deposition rate at least one order of magnitude higher than atomic layer deposition was achieved on a large carrier surfaces without significantly reducing the resultant passivation quality.
95(2009); http://dx.doi.org/10.1063/1.3246159View Description Hide Description
Plasma focus research in the direction of fusionenergy faces the limitation of observed neutron saturation; the neutron yield falls away from , the scaling deteriorating as storage energy increases toward 1 MJ. Numerical experiments confirm that applies at low energies and drops to toward 25 MJ; deteriorating already at several hundred kilojoules. We point out that the cause is the dynamic resistance of the axial phase that is constant for all plasma foci. This dynamic resistance dominates the circuit as capacitor bank surge impedance becomes insignificant at large , causing current, hence neutron “saturation.”
Diagnostic based modeling for determining absolute atomic oxygen densities in atmospheric pressure helium-oxygen plasmas95(2009); http://dx.doi.org/10.1063/1.3242382View Description Hide Description
Absolute atomic oxygen ground state densities in a radio-frequency driven atmospheric pressure plasma jet, operated in a helium-oxygen mixture, are determined using diagnostic based modeling. One-dimensional numerical simulations of the electron dynamics are combined with time integrated optical emission spectroscopy. The population dynamics of the upper atomic oxygen state is governed by direct electron impact excitation, dissociative excitation, radiation losses, and collisional induced quenching. Absolute values for atomic oxygen densities are obtained through comparison with the upper state. Results for spatial profiles and power variations are presented and show excellent quantitative agreement with independent two-photon laser-induced fluorescence measurements.